As far as the planning thing - you don't need ANY money to plan something. You just need to get together to talk about it.
Guns are cheap (or free if you know where to look), and you can smuggle just about anything onto a plane if you try hard enough. Besides, ceramic glocks (terrorist weapon of choice) are not detectable by normal means.
And as far as the bomb thing afterwards - still UNCONFIRMED by legitimate sources. My guess, aircraft probably broke off a wing or two and was driven through to upper or lower floors. Once the flames finally reached, the wing-board fuel tanks on other floors exploded.
Of course, I am purely speculating, but then again, so are you.
Plus, reports of major gas leaks all over Manhattan, could easily have had gas pockets within the building explode.
Remember, everybody is just speculating right now. The only people who know exactly what happened are either dead or hiding.
I don't doubt that it was well planned, but you have to admit, noone knows who did it (though we have a few guesses).
After all, terrorists generally don't have inordinate amounts of money to play with...
SunnyD